Bird Poop in My Hair

I ride under a lot of tree branches every day. About half of my ten minute ride to work is under tree branches. These tree branches are filled with many birds, and these birds are pooping all day long. Which raises the question: After years of riding my bike to and from work every day, how much bird poop must fall directly on me without my knowing it? How much bird poop do I have in my hair right now?